26.13: Genetic Code

The Human Genome Project started in 1990 with the lofty goal of sequencing the complete set of human DNA. This project was completed in April 2003, ahead of schedule and under the budget set for it (a rare occurrence for a government project). With this knowledge, we can now identify genetic disorders quickly and personalize treatment for many diseases. However, much work still remains to fully understand the connections between specific DNA sequences and specific diseases.

The Genetic Code

Each particular organism contains many protein molecules that are specific to that organism. The particular base sequence of DNA is responsible for the production of all of the different proteins that are present in each and every living thing that has ever inhabited the Earth. How does that work? Cells use the unique sequence of DNA bases to decide which proteins to synthesize. A gene is a segment of DNA that carries a code for making a specific polypeptide chain. The cell essentially decodes the DNA in order to make whatever peptides and proteins are needed by that organism.

The genetic code works as a series of three-letter codes. Each sequence of three letters, called a triplet, corresponds to one of the twenty common amino acids. The triplets are read by the cell, one after the other, in the process of protein synthesis. The table below shows all of the possible triplets and the amino acids that result from each three-letter code.

Table 26.13.1: DNA Triplet Codes for Amino Acids

AAA

Lys

GAA

Glu

TAA

Stop

CAA

Gln

AAG

Lys

GAG

Glu

TAG

Stop

CAG

Gln

AAT

Asn

GAT

Asp

TAT

Tyr

CAT

His

AAC

Asn

GAC

Asp

TAC

Tyr

CAC

His

AGA

Arg

GGA

Gly

TGA

Stop

CGA

Arg

AGG

Arg

GGG

Gly

TGG

Trp

CGG

Arg

AGT

Ser

GGT

Gly

TGT

Cys

CGT

Arg

AGC

Ser

GGC

Gly

TGC

Cys

CGC

Arg

ATA

Ile

GTA

Val

TTA

Leu

CTA

Leu

ATG

Met

GTG

Val

TTG

Leu

CTG

Leu

ATT

Ile

GTT

Val

TTT

Phe

CTT

Leu

ATC

Ile

GTC

Val

TTC

Phe

CTC

Leu

ACA

Thr

GCA

Ala

TCA

Ser

CCA

Pro

ACG

Thr

GCG

Ala

TCG

Ser

CCG

Pro

ACT

Thr

GCT

Ala

TCT

Ser

CCT

Pro

ACC

Thr

GCC

Ala

TCC

Ser

CCC

Pro

The DNA code word GCA corresponds to the amino acid arginine, while the DNA code word TCG corresponds to the amino acid serine. Most amino acids are represented by more than one possible triplet code, but each triplet code yields only one particular amino acid. Three of the DNA code words (TAA, TAG, and TGA) are stop or termination code words. The translation of a DNA base sequence begins with a start code word and runs until a stop code word is reached.

Even with only four different bases, the number of possible nucleotide sequences in a DNA chain is virtually limitless. The particular DNA sequence of a particular organism constitutes the genetic blueprint for that organism. This genetic blueprint is found in the nucleus of each cell of the organism and is passed on from parents to offspring. The incredible diversity of life on Earth stems from the differences in the genetic code of every living thing.

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